The light structural materials, such as magnesium alloys, are attractive for applications including the transportation industry, power tools, sporting goods, and computer and electronic products. The magnesium components have better strength-to-weight ratio than their aluminum or steel counterparts, thereby reducing the total vehicle weight and loading and improving fuel economy, while also increasing safety, significantly lowering emissions, and increasing recyclability. Although various casting processes are used to produce magnesium alloy parts, around 90% of cast magnesium components are produced by high-pressure die casting process. Other relevant production technologies include sand casting, permanent mold and investment casting, as well as squeeze casting, and varies types of semi-solid casting technologies. All commercial high-pressure die casting magnesium alloys are based on Mg—Al—Mn system with additions of Zn, Si, or rare earth elements (RE).
Die casting magnesium alloys of Mg—Al—Mn system, such as AM50A and AM60B, and of Mg—Al—Zn system, such as AZ91D, exhibit good castability, good corrosion resistance, and combination of ambient strength and ductility; however, they exhibit poor elevated temperature strength, poor creep resistance, and poor bolt load retention capability.
On the other hand, Mg—Al—Si alloys, such as AS41, AS31 and AS21, and Mg—Al—Re alloys, such as AE42, AE43 and AE44, exhibit improved creep resistance but reveal inadequate corrosion performance (Mg—Al—Si alloys) or poor castability (AE42 and AE43 alloys). In addition, both AS and AE alloy-series exhibit relatively low tensile yield strength and fatigue strength at room temperature.
Recently several creep-resistant magnesium alloys were developed and described, for example in U.S. Pat. No. 6,139,151, EP 1,135,630, EP 1,127,950, U.S. Pat. Nos. 6,342,180, 6,264,763 and 7,041,179. These alloys are based on Mg—Al system with addition of Ca or Ca+RE as the main alloying elements to increase creep resistance. On the other hand, another alkaline-earth element, Sr, was mainly used as a minor ingredient for addition to Mg—Al—Ca or Mg—Al—Ca—Re systems.
Another approach was recently demonstrated in U.S. Pat. Nos. 6,322,644 and 6,808,679 that describe magnesium based alloys, which contain 2 to 9 wt % aluminum, 0.5 to 7 wt % strontium, 0 to 0.35 wt % zinc and 0.0 to 0.60 wt % manganese.
EP 1418247 discloses a magnesium based alloy for high-pressure die casting containing 4.0 to 9.0 wt % aluminum, 0.5 to 4 wt % strontium, and 0.03 to 2.5 wt % barium. The alloy exhibits an adequate creep resistance, but barium is considered as a very toxic element, and its use is undesirable.
It is an object of this invention to provide magnesium-based alloys being suitable for high temperature applications, and having good corrosion resistance.
It is another object of this invention to provide alloys which are particularly well adapted for high-pressure die casting process, and which exhibit improved castability.
It is a further object of this invention to provide alloys which may also be used for other applications, such as thixoforming, sand casting, permanent mold casting, and squeeze casting.
It is a still further object of this invention to provide alloys, which are not prone to die sticking and hot cracking.
It is a still further object of this invention to provide alloys with improved strength at ambient and elevated temperatures, as well as with improved creep resistance at elevated temperatures up to at least 175° C.
It is a still further object of this invention to provide alloys which demonstrate the aforesaid behavior and properties, and also have a relatively low cost.
Other objects and advantages of present invention will appear as description proceeds.